Martensitic stainless steel, method for making parts from said steel and parts thus made

ABSTRACT

Martensitic stainless steel, characterised in that it comprises, in percentages by weight:
         0.22%≦C≦0.32%   0.05%≦N≦0.15%, with 0.33%≦C+N≦0.43%   10%≦Cr≦12.4%   0.10%≦V≦0.40%   0.10%≦Mo≦1.0%   trace levels≦Ni≦1.0%   trace levels≦Mn≦1.0%   trace levels≦Si≦1.0%   trace levels≦W≦1.0%   trace levels≦Co≦1.0%   trace levels≦Cu≦1.0%   trace levels≦Ti≦0.010%   trace levels≦Nb≦0.050%   trace levels≦Al≦0.050%   trace levels≦S≦0.020%   trace levels≦O≦0.0040%   trace levels≦P≦0.03%   trace levels≦B≦0.0050%   trace levels≦Ca≦0.020%   trace levels≦Se≦0.010%   trace levels≦La≦0.040%   trace levels≦Ce≦0.040%
 
the remainder being iron and impurities resulting from the production operation.
       

     Method for producing a component from this steel, and component obtained in this manner, such as a mould element for the production of articles of plastics material.

The present invention relates to steelmaking, more precisely martensiticstainless steels which are intended, for example, for the production ofmoulds for producing plastics materials by means of injection.

In order to produce moulds for material injection, the industry usesstainless steels of the family AISI 420 which have a chromium content offrom 12 to 15% (in percentages by weight, as is the case for all thecontents indicated in the remainder of the text), a silicon content ofless than 1%, a manganese content of less than 1%, a carbon content offrom 0.16 to 0.45% and a nitrogen content which is the one which resultsnaturally from the production operation and which is generally up to0.03%. Generally, the content of vanadium does not exceed 0.1% and isthe result of simple fusion of the raw materials. In the same manner,the content of molybdenum is the result of the fusion of the rawmaterials and does not exceed 0.2%, unless from 0.2 to 1.0% is added inorder to improve the corrosion resistance. More specifically, the steelwith the reference X40Cr14 which is capable, owing to its carbon contentof from 0.36 to 0.45%, of exceeding a hardness of 50 HRC, providessignificant abrasion resistance.

Taking into account the application envisaged, the effectiveness of thematerial must be evaluated by obtaining a good compromise between thefollowing properties:

-   -   the wear resistance desired in order to be able to produce the        maximum number of components with guaranteed geometric        regularity, including with plastics materials which are made        abrasive by the integration of fibres or other reinforcement        additives; this wear resistance is provided by a high level of        hardness;    -   sufficient toughness to prevent fractures during thermal        processing, assembly/disassembly operations, or service; for        these steels which are particularly brittle, this property is        found to contradict the above one, the toughness decreasing as        the hardness increases;    -   good polishability which allows a surface polishing quality to        be readily achieved on the surface of the mould in order to        produce components of plastics materials with a smooth and        uniform surface appearance; the steel must also be able to        maintain this polished state for as long as possible,    -   sufficient corrosion resistance to prevent pitting, tarnishing,        alteration of the polish state during storage of the moulds and        during service, in the context of production of plastics        materials which are slightly or moderately chemically        aggressive; the most active substances, for example, by means of        salting out chloride ions, require steels or alloys from other        families.

After machining a blank to approximate dimensions of the final shape,the moulds are subjected to the following thermal processing operationin an oven under a controlled atmosphere:

-   -   raising to quenching temperature in the range of from 1000 to        1050° C. followed by maintaining in this range for some tens of        minutes,    -   quenching under gas pressure down to a temperature in the order        of 80° C.;    -   raising the temperature for two tempering cycles.

Two temperature ranges are generally proposed for tempering:

-   -   low temperature tempering: 150 to 250° C.,    -   tempering at 490/530° C. in the secondary hardening zone of the        steel.

Theoretically, the two successive tempering operations are both carriedout in the same range.

Careful consideration must be given as to the precise selection of theprocessing parameters.

For the quenching, it is metallurgically recommended to seek highquenching temperatures in order to achieve a favourable martensiticmicrostructure. However, high quenching temperatures promote deformationand produce residual stresses which are capable of leading to fractures.In practice, the gas pressures are limited to values of from 2 to 4 bar.

When quenching stops, before continuing with the tempering operations,fractures are possible if the cooling continues down to ambienttemperature. However, the usual selection of stopping the cooling atapproximately 80° C. involves the risk of retaining residual austenite,in particular if the subsequent tempering operations are fixed below500° C., and consequently not being able to obtain the desired nominalhardness.

For the tempering operations, the selection of low temperatures onlyallows the constraints to be partially overcome, and if the compositionof the steel and the quenching cycle have allowed residual austenite toremain, with the tempering operation not decomposing it, the desiredhardness is not achieved. High-temperature tempering operationsdecompose the austenite and relax the residual stresses, but reduce thestrength and the corrosion resistance.

There is also the problem of the cost of these steels, owing to the highcontents of alloy elements which they require and which it would benecessary to be able to minimise without impairing the desiredproperties.

The object of the invention is to define an economic steel compositionfor applications involving moulds for the production of articles ofplastics materials which have, with respect to the references AISI 420and X40Cr14, the following properties:

-   -   preferential equivalent hardness of from 49 to 55 HRC in the        processed state in order to withstand abrasion;    -   equivalent corrosion resistance;    -   improved strength with equal hardness;    -   improved polishability;    -   and to have these properties under industrial thermal processing        conditions which are comparable with conventional conditions.

To this end, the invention relates to a martensitic stainless steel,characterised in that it comprises, in percentages by weight:

-   -   0.22%≦C≦0.32%    -   0.05%≦N≦0.15%, with 0.33%≦C+N≦0.43%    -   10%≦Cr≦12.4%    -   0.10%≦V≦0.40%    -   0.10%≦Mo≦1.0%    -   trace levels≦Ni≦1.0%    -   trace levels≦Mn≦1.0%    -   trace levels≦Si≦1.0%    -   trace levels≦W≦1.0%    -   trace levels≦Co≦1.0%    -   trace levels≦Cu≦1.0%    -   trace levels≦Ti≦0.010%    -   trace levels≦Nb≦0.050%    -   trace levels≦Al≦0.050%    -   trace levels≦S≦0.020%    -   trace levels≦O≦0.0040%    -   trace levels≦P≦0.03%    -   trace levels≦B≦0.0050%    -   trace levels≦Ca≦0.020%    -   trace levels≦Se≦0.010%    -   trace levels≦La≦0.040%    -   trace levels≦Ce≦0.040%        the remainder being iron and impurities resulting from the        production operation.    -   Preferably 0.08%≦N≦0.12%.    -   Preferably 11.0%≦Cr≦12.4%.    -   Preferably 0.15%≦V≦0.35%.    -   Preferably trace levels≦Si≦0.5%.    -   Preferably 0.10%≦Mo+W/2≦1.20%.    -   Preferably trace levels≦Ti≦0.003%.    -   Preferably trace levels≦Nb≦0.010%.    -   Preferably trace levels≦O0.0015%.    -   Preferably trace levels≦S≦0.003%.    -   Preferably trace levels≦Mn+Cu+Co≦1.8%.

The invention also relates to a method for producing a component ofmartensitic stainless steel, characterised in that:

-   -   a steel of the above type is produced, cast, forged or rolled        and annealed;    -   the steel is machined in order to confer thereon the shape of        the component;    -   the machined steel is austenitised at a temperature of from        990-1040° C., preferably 1000-1030° C.;    -   the austenitised steel is quenched at a rate of between 10 and        40° C./min in the temperature range of from 800 to 400° C.;    -   two tempering operations are carried out on the quenched steel,        in order to confer thereon its final hardness.

The tempering operations can each be carried out at a temperature offrom 200 to 400° C., preferably from 300 to 380° C. for a minimum of 2hours whilst ensuring that the nominal temperature is maintained in thecore for at least 1 hour, in order to obtain a hardness of from 49 to 55HRC.

The tempering operations can each be carried out at a temperature offrom 530 to 560° C. for a minimum of 2 hours whilst ensuring that thenominal temperature is maintained in the core for at least 1 hour, inorder to obtain a hardness of between 42 and 50 HRC.

The invention also relates to a component of martensitic stainlesssteel, characterised in that the element produced using the method isproduced in accordance with the preceding method.

This may be a mould element which is intended for the production ofarticles of plastics materials.

As will be appreciated, the invention is based on a steel compositionwhose contents of carbon and chromium are simultaneously at the lowerend of the ranges generally required, or even sometimes below in thecase of the chromium content, with the imposition of precise conditionson other elements which are present or which must be limited or avoided.A production method is associated with this composition.

The inventors concentrated on actually taking into consideration theproperties of the steel following the production operation, and inparticular the industrial processing as described above, and not inaccordance with laboratory conditions. The research was carried out withthe intent of optimising the action of the alloy elements in order tolimit the quantity thereof introduced.

The main considerations which have led to the invention are as follows.

The polishability and the surface quality of the polished state of thesteel are degraded by:

-   -   the presence of non-metal oxide inclusions which do not reflect        light and which further disintegrate or become bare in contact        with abrasives, and form leaving streaks or “comets tails” at        the surface of the mould;    -   the interdendritic segregations which form naturally during        solidification of the bar and generate on the surface of the        moulds harder zones or lines which alternate with softer zones        or lines, bringing about undulations during polishing, owing to        the fact that the soft zones yield more quickly than the hard        zones;    -   the presence of non-dissolved micrometric chromium carbides        during the quenching operation.

Generally, the strength, which is moderate for this family of steels,becomes lower for a given hardness, as the chromium content increases.It could be improved by balancing the composition, in particular withadditions of nickel and manganese which allow a residue of austenite tobe retained during the quenching operation. This solution which furtherno longer has an effect if the tempering operations are carried outabove 500° C. is, however, found to be unstable and impedes theproduction of the hardness. It has not been retained, particularly sinceit was not compatible with the desired reduction of the content of alloyelements.

In order to achieve the set objectives, it has been decided:

-   -   on the one hand to produce the steel in accordance with known        methods which limit the presence of oxidised non-metal        inclusions, therefore conferring on the steel a low content of        O,    -   and on the other hand to globally reduce the alloy elements,        introduce nitrogen, and optimise the equilibriums between        elements in order to increase the strength, reduce the        interdendritic segregation and limit the density of micrometric        precipitates.

The invention will be better understood from the following description,given with reference to the following appended Figures:

FIG. 1 which illustrates micrographs of samples of a reference steel andtwo steels according to the invention, illustrating the density and thedistribution of the micrometric carbides in the state for use of thesesteels;

FIG. 2 which illustrates the influence of the temperature of the twotempering operations on the corrosion resistance of a steel according tothe invention;

FIG. 3 which illustrates the influence of the temperature of thetempering operations on the corrosion resistance;

FIG. 4 which illustrates the interactions of the content of Cr and thequenching rate on the corrosion resistance;

FIG. 5 which illustrates the hardness of the steels according to theinvention and a reference steel in accordance with the temperature ofthe tempering operations.

Table 1 sets out the compositions of the samples examined. The sample“reference” corresponds to a steel of the conventional type X40Cr14. Thesamples Exp. 1 to Exp. 7 are not in accordance with the invention butallow the disadvantages to be overcome which are involved in notcomplying with all the conditions required by the invention. The samplesInv. 1 and Inv. 2 are in accordance with the invention.

TABLE 1 Chemical composition of the steels examined C N Si Mn Ni Cr Mo VCo Cu Al W Nb S P Ti O Identification % % % % % % % % % % % % % ppm ppmppm ppm Reference 0.41 0.02 0.38 0.50 0.27 14.56 0.29 0.06 0.04 0.080.023 0.04 0.01 5 210 34 12 Exp. 1 0.27 0.11 0.27 0.40 0.32 14.17 0.160.14 0.02 0.06 0.019 0.02 0.01 2 170 20 7 Exp. 2 0.27 0.12 0.28 0.370.32 10.71 0.16 0.13 0.03 0.03 0.015 0.07 0.02 3 150 12 11 Exp. 3 0.270.11 0.32 0.42 0.31 10.75 0.15 0.01 0.01 0.02 0.012 0.05 0.03 4 140 30 8Exp. 4 0.26 0.12 0.30 0.40 0.31 10.80 0.15 0.36 0.01 0.06 0.019 0.070.02 2 150 28 8 Exp. 5 0.23 0.13 0.31 0.42 0.32 12.75 0.15 0.14 0.040.07 0.021 0.02 0.02 2 190 18 7 Exp. 6 0.27 0.11 0.29 0.42 1.04 12.890.15 0.13 0.02 0.09 0.010 0.02 0.01 4 160 16 10 Exp. 7 0.28 0.11 0.270.42 0.35 10.89 0.89 0.13 0.01 0.02 0.013 0.02 0.01 3 170 12 11 Inv 10.28 0.10 0.38 0.46 0.32 12.38 0.29 0.18 0.03 0.03 0.014 0.03 0.01 1 17013 9 Inv 2 0.27 0.12 0.37 0.43 0.32 11.04 0.32 0.22 0.04 0.01 0.013 0.010.02 1 200 16 7

An object of the invention is therefore to provide an optimised steelwhich is intended to be processed in accordance with the range ofindustrial quenching rates, preferably with a subsequent dual temperingoperation at low temperature (<400° C.) for a hardness of 52 HRC with astrength and a corrosion resistance which are equal to or greater thanthose of the reference steel AISI 420 or X40Cr14 in its usualapplication.

Furthermore, an object of the invention is to limit to the greatestpossible extent the additions of alloy elements, in particular metalelements, in order to reduce the production cost, prevent the presenceof residual austenite after the quenching operation and reduce theextent of interdendritic segregation which is detrimental to thestrength and the quality of the polishing.

To this end, the inventors have arrived at the following resultsrelating to the definition of the composition of the steels of theinvention.

The nitrogen content must be between 0.05% and 0.15% and preferablybetween 0.08% and 0.12%. This element is therefore systematicallypresent with a high content since it is indispensable in order to formcarbonitrides of the type V(C,N) which are capable of preventing grainenlargement during austenisation after the chromium carbides havedissolved. However, an excessive content would be detrimental ifexceeding the solubility limit in the solid state and would be a sourceof metallurgical defects. The nitrogen associates with the carbon inorder to confer the hardness and is involved in the corrosionresistance. The content of nitrogen can be adjusted by insufflation ofgaseous nitrogen when the liquid steel is produced.

Carbon contributes mainly to conferring the hardness required,associated with the nitrogen. Taking into account the hardness requiredafter tempering at low temperature, the percentage must be between 0.22%and 0.32%. Furthermore, the total C+N must be between 0.33% and 0.43% inorder to allow the desired hardness to be achieved after tempering.

Chromium confers on the steel the corrosion resistance thereof. Takinginto account the industrial quenching rates used, and the temperingrange selected, and in accordance with the mechanisms set out above, thecontent thereof must be between 10 and 12.4% and preferably between 11.0and 12.4%.

Vanadium must be present at a content of between 0.10% and 0.40% andpreferably between 0.15% and 0.35%. The presence thereof isindispensable for forming with the carbon and the nitrogen a sufficientdensity of micro- and nano-precipitates which are capable of preventingthe grain enlargement. An excessive content would be detrimental owingto the excessive fixing of carbon which would impair the hardening andowing to the formation, during solidification, of carbides which areisolated or in a cluster which are unfavourable for the strength and thepolish state quality.

Molybdenum complements the action of the chromium for the corrosionresistance; it is present owing to the recycling operations or byintentional addition, at percentages of between 0.10 and 1.0%. A greatercontent would be detrimental owing to the increase in the extent of theinterdendritic segregation and owing to the risk of forming deltaferrite.

Nickel may be present at contents of less than 1.0%, in particular owingto the contribution by the raw materials. No beneficial effect of anaddition within this limit has been found in terms of the toughness.However, a greater content would be capable of maintaining the residualaustenite in the processed state.

Manganese is an element which is naturally present in this family ofsteel owing to the production methods and the raw materials available.No beneficial effect has been found and it has been found to benecessary to limit the concentration thereof to 1.0% in order to preventresidual austenite after thermal processing.

Silicon is naturally present for the production and the deoxidation ofthe steel. The content thereof must be limited to 1.0% and preferably0.5% since it acts on the process of solidification and the delta/gammaconversion and consequently can bring about the presence of deltaferrite or local segregations owing to the presence of this phase at theend of solidification before forging.

Tungsten may be present at contents of less than 1.0% without having anyfavourable or detrimental effect on the product. Nonetheless, owing toits individual action or synergy with molybdenum, it may promote thepresence of delta ferrite in the state for use or local precipitationsor segregations resulting from the presence of delta ferrite at anystage of the thermomechanical process. It will be preferable to complywith the condition 0.10%≦Mo+W/2≦1.20%.

Cobalt and copper have no beneficial effect which has been identifiedbut may be present at contents of less than or equal to 1.0%: highercontents could promote the presence of residual austenite.

It is preferable for the total of the contents of Mn, Cu and Co to be≦1.8% in order to limit the risks of the presence of residual austenite.

Titanium and niobium are very reactive elements which form very hardprecipitates which are detrimental to the quality of polish state. Thecontent thereof must be kept as low as possible: a maximum of 0.010%,preferably a maximum of 0.003% for Ti and a maximum of 0.050%,preferably a maximum of 0.010% for Nb.

The aluminium added for the deoxidation of the steel may remain presentin oxide inclusions which are very detrimental to the polish state. Thelevel of addition must be adapted to the production methods used. Amaximum content of 0.050% is tolerable, on condition that it does notlead to the presence of inclusions of aluminium oxide or silicoaluminates in large quantities which would lead to the acceptablecontent of O being exceeded (0.0040%, preferably 0.0015%).

Sulphur is preferably limited to a content of less than 0.003% in orderto prevent the formation of sulphur inclusions. Optionally, however, itis possible to elect to carry out a voluntary addition in the range offrom 0.003 to 0.020% associated preferably with another element (Se upto 0.010%, Ca up to 0.020%, La up to 0.040%, Ce up to 0.040%) promotingthe formation of globular sulphides in order to improve themachinability, to the detriment to a certain extent of the quality ofthe polish state.

The maximum content of oxygen is 0.0040%, preferably 0.0015%. Thiselement is an indicator of the inclusion density, which is detrimentalto the polish state of the surface when it is too high. This contentmust be kept as low as possible, and the production method of the steelmust be selected as a result. In practice, known methods allow values aslow as O=5 ppm under economically acceptable conditions.

The content of phosphorus is limited to 0.03% which is a common contentin this class of steels. No detrimental effect of P has been noted inthis range.

Boron can be added in order to improve the quenchability, at a contentwhich does not exceed 0.0050%.

The preferable contents indicated for some elements may be imposed aloneand not necessarily in combination with the other preferred contentsindicated.

The non-cited elements may be present at contents of the level ofimpurities resulting from the production which do not modify theproperties which the invention seeks to optimise.

The products must be produced in accordance with the provisions of theprevailing art for special high-quality steels which are intended forthe applications for moulding articles of plastics materials with theobjective of limiting the content of inclusions and the segregation inorder to obtain a high quality polish state. The production mustcomprise, after fusion, a step for deoxidation and elimination of theinclusions in a metallurgical reactor. Preferably, in particular for theproduction of large moulds and in order to obtain the highest qualitiesof polish state, a remelting operation using a consumable electrodeunder a slag will be carried out in order to improve the inclusionpurity and distribute the alloy elements, and in particular nitrogen ina homogeneous manner in the entire mass.

A thermomechanical transformation by means of forging or rollingfinishing with an annealing operation must follow in order to complementthe homogeneity and the compactness of the microstructure.

After machining the component to the final shape and before operation,the products must, in accordance with the preferred operating method, besubjected to a thermal processing operation which comprisesaustenisation at approximately 1020° C. (from 990 to 1040° C.,preferably 1000-1030° C.), a controlled quenching operation, forexample, under neutral gas pressure, at a rate of between 10 and 40°C./minute adapted to the size of the component, then two temperingoperations at a temperature of from 200 to 400° C., preferably between300 and 380° C., in order to obtain a hardness of approximately 52 HRC±2HRC and generally between 49 and 55 HRC.

Optionally, for applications which do not require a hardness greaterthan 50 HRC, the steel defined by the invention could be processed bymeans of dual tempering at 530° C. to 560° C. for hardnesses which areless than or equal to 50 HRC and greater than or equal to ±42 HRC, underwhich conditions the corrosion resistance is found to be adequate.

For the reference steel, the chromium carbides (M₂₃C₆) which exist inthe delivery state are dissolved during the austenisation which precedesthe quenching operation, and the temperature to be maintained is limitedto 1020/1030° C. in order to prevent grain growth. However, at thisdissolution temperature, a significant quantity of carbides which aredistributed in a heterogeneous manner remains. By substituting fromapproximately 0.10 to 0.15% of the content of carbon with nitrogen, areduction of approximately 2% of the content of chromium and asimultaneous introduction of vanadium, it is observed at the adequatequenching temperature that the grain which is fixed by means ofnanometric precipitates of vanadium carbonitrides V(C,N) does not growwhilst the majority of the chromium carbides are dissolved.

For three of the compositions examined, the compared calculation of theequilibriums at 1030° by means of thermodynamic simulation using thesoftware THERMOCALC (commonly used by metallurgists) illustrates thismutation (see Table 2).

TABLE 2 Calculation of the thermodynamic equilibriums at 1015° C. forthree representative compositions Molar % Nominal composition of Molar %Matrix composition Identification C (%) N (%) Cr (%) Mo (%) V (%) M23C6of V(C,N) Cr (%) Mo (%) N (%) P.R.E. Reference 0.41 0.02 14.56 0.29 0.062.84 0.00 13.45 0.26 0.02 15.0 Inv. 1 0.28 0.10 12.38 0.29 0.18 0.000.20 12.35 0.29 0.09 16.0 Inv. 2 0.27 0.12 11.04 0.32 0.22 0.00 0.2611.00 0.32 0.10 15.1

The effective density of the micrometric carbides observed on industrialproducts and illustrated in FIG. 1 effectively decreases in asignificant manner between the reference composition and thecompositions of the invention, which constitutes a favourable factor forthe quality of the polish state.

For the reference steel, the capacity for corrosion resistance is,theoretically in accordance with basic knowledge, above all linked tothe content of chromium available in the matrix; the thermodynamiccalculations show that the carbides which are not dissolved duringaustenisation fix approximately 0.9% of chromium. This quantity ofchromium which is not available for the corrosion resistance becomeslower than 0.1% for the experimental grades which are alloyed withvanadium and nitrogen. In accordance with the following formula:

P.R.E. (Pitting Resistance Equivalent)=% Cr+3.3×% Mo+30×% N

which conventionally allows the compositions to be classified inaccordance with their resistance to pitting and applied to the effectivecomposition of the matrix, it is found according to Table 2 that theexperimental compositions Inv 1 and 2 have a coefficient which is closeto that of the reference.

In addition to the considerations set out above expressing a potentialin the crude quenching state, it is advantageous to carry outmeasurements in the effective state of the metal at the stage of use.The electrochemical method carried out from the standard ASTM G 108involves polarising the sample for 15 minutes in an aqueous solution ofH₂SO₄ at 1% by weight, at a potential of −550 mV/ECS then carrying out ascanning operation back and forth at 60 mV/mn from −550 mV to +500 mV.The characteristic lines intensity/potential on return may have twopeaks, one (peak 1) owing to the dissolution of the matrix, the second(peak 2), at a higher potential, linked to the dissolution in the regionof the precipitates of chromium carbides. The steel becomes moresensitive to corrosion as the dissolution current becomes more intense.Characteristic lines are set out in FIGS. 2 and 3.

According to current practices for reference steels, whilst obtainingthe desired hardness of approximately 52 HRC, two parameters of thethermal processing are found to be influential for the corrosionresistance: the tempering temperature and the quenching rate.

These effects have been set out by laboratory tests:

a) Tempering Effect:

FIG. 2 illustrates for the casting INV1 that the steel becomes highlysensitive with respect to corrosion for tempering operations which arecarried out in the hardening zone of approximately 500° C. If thecorrosion resistance is a characteristic which must imperatively beprioritised for the applications envisaged, low-temperature temperingoperations will therefore be preferred (200-380° C.).

This tendency is confirmed for all the compositions tested, asillustrated in FIG. 3. This shows the influence of a dual temperingoperation of 2 hours at 380° C., or at a temperature close to 500° C.,on the corrosion resistance taking into account the corrosion current atthe peak 2 of FIG. 2. The precise temperature of the dual temperingoperations at approximately 500° C. has been adjusted so that it allowsthe same hardness to be obtained as after a dual tempering operation at380° C. It has been found in particular that the sample according to theinvention has a corrosion resistance which is very comparable to that ofthe reference sample for a dual tempering operation at 380° C.

For low tempering temperatures, it has further been verified that thecorrosion resistance decreases slightly between 200° C. and 380° C. anddegrades rapidly above 400° C.

So that the tempering operations have the intended effect, they mustlast at least 2 hours, and their nominal temperature must be maintainedat the core of the component for at least 1 hour.

b) Effect of the Quenching Rate:

Unexpectedly, as illustrated in FIG. 4 which compares two experimentalcastings which are distinguished from each other only by their chromiumcontent, the increase of the content of this element does not improvethe corrosion resistance under industrial quenching conditions with acooling rate in the order of 20° C./minute in the range 900/400° C. Thelow cooling rate brings about the development of the peak 2 whichindicates the precipitation of carbides or nitrides and whose extentbecomes greater as the content of chromium increases and amplifies thecorrosion current of the matrix (peak 2).

These results are confirmed for the various compositions examined.

According to the invention, a quenching rate is selected which iscompatible with the technical knowledge of the thermal processingoperation and between 10 and 40° C./min in the temperature range from800 to 400° C.

In conclusion, in the context of an industrial quenching operation thebest corrosion resistance is obtained with the low-temperature temperingoperations and, in this configuration, the variation of the chromiumcontent in the range from 10.5 to 15% does not confirm the beneficialeffect normally recognised for this alloy element.

The same unfavourable effects of reducing the quenching rate andincreasing the tempering temperature are found with respect to thetoughness. This property is commonly simply appreciated from theconventional mechanical characteristics of elongation and reduction ofarea during the traction and impact flexion energy test on non-notchedbars having dimensions of 55×10×7 mm. For the tests concerned, aquenching operation of 16° C./min was carried out on all the samplesthen a dual tempering operation of 2 hours. The results set out in Table3 illustrate:

-   -   for the composition Inv. 2, taken as an example, the negative        effect of reducing the quenching rate;    -   the embrittling effect of the dual tempering operation at        approximately 500° C.;    -   for the dual tempering operation at low temperature (380° C.),        the superior nature of the hardness/toughness compromise of the        two steels of the invention with respect to the reference.

TABLE 3 Mechanical characteristics measured for three compositions onsamples representative of industrial products Dual tempering operationof 2 hours at 380° C. Dual tempering operation of 2 hours at hightemperature Impact flexion Impact flexion Quenching Non-notchedNon-notched rate: Hardness A Z sample Tempering Hardness A Z sampleComposition (° C./min) HRC % % J ° C. HRC % % J Reference 16 51.3 4.5 9110 500 51.3 3.0 3 32 Inv. 1 16 53.2 7.5 21 210 520 53.0 3.5 5 38 Inv. 216 53.9 6.5 17 130 520 52.3 6.0 17 35 Inv. 2 60 53.8 9.5 35 260

The compositions of the invention allow the hardness of 52 HRC or aboveto be obtained after quenching under industrial conditions and dualtempering at 380° C., in spite of the softening occurring in this rangefor this family of steels from the crude quenching material, asillustrated in FIG. 5.

1. Martensitic stainless steel, characterised in that it comprises, inpercentages by weight: 0.22%≦C≦0.32% 0.05%≦N≦0.15%, with 0.33%≦C+N≦0.43%10%≦Cr≦12.4% 0.10%≦V≦0.40% 0.10%≦Mo≦1.0% trace levels≦Ni≦1.0% tracelevels≦Mn≦1.0% trace levels≦Si≦1.0% trace levels≦W≦1.0% tracelevels≦Co≦1.0% trace levels≦Cu≦1.0% trace levels≦Ti≦0.010% tracelevels≦Nb≦0.050% trace levels≦Al≦0.050% trace levels≦S≦0.020% tracelevels≦O≦0.0040% trace levels≦P≦0.03% trace levels≦B≦0.0050% tracelevels≦Ca≦0.020% trace levels≦Se≦0.010% trace levels≦La≦0.040% tracelevels≦Ce≦0.040% the remainder being iron and impurities resulting fromthe production operation.
 2. Steel according to claim 1, characterisedin that 0.08%≦N≦0.12%.
 3. Steel according to claim 1, characterised inthat 11.0%≦Cr≦12.4%.
 4. Steel according to claim 1, characterised inthat 0.15%≦V≦0.35%.
 5. Steel according to claim 1, characterised in thattrace levels≦Si≦0.5%.
 6. Steel according to claim 1, characterised inthat 0.10%≦Mo+W/2≦1.20%.
 7. Steel according to claim 1, characterised inthat trace levels≦Ti≦0.003%.
 8. Steel according to claim 1,characterised in that trace levels≦Nb≦0.010%.
 9. Steel according toclaim 1, characterised in that trace levels≦O≦0.0015%.
 10. Steelaccording to claim 1, characterised in that trace levels≦S≦0.003%. 11.Steel according to claim 1, characterised in that trace levelsMn≦+Cu+Co≦1.8%.
 12. Method for producing a component of martensiticstainless steel, characterised in that: a steel according to claim 1 isproduced, cast, forged or rolled and annealed; the steel is machined inorder to confer thereon the shape of the component; the machined steelis austenitised at a temperature of from 990-1040° C., preferably1000-1030° C.; the austenitised steel is quenched at a rate of between10 and 40° C./min in the temperature range of from 800 to 400° C.; twotempering operations are carried out on the quenched steel, in order toconfer thereon its final hardness.
 13. Method according to claim 12,characterised in that the tempering operations are each carried out at atemperature of from 200 to 400° C., preferably from 300 to 380° C. for aminimum of 2 hours whilst ensuring that the nominal temperature ismaintained in the core for at least 1 hour, in order to obtain ahardness of from 49 to 55 HRC.
 14. Method according to claim 12,characterised in that the tempering operations are each carried out at atemperature of from 530 to 560° C. for a minimum of 2 hours whilstensuring that the nominal temperature is maintained in the core for atleast 1 hour, in order to obtain a hardness of between 42 and 50 HRC.15. Component of martensitic stainless steel, characterised in that theelement produced using the method is produced in accordance with claim12.
 16. Component according to claim 15, characterised in that it is amould element which is intended for the production of articles ofplastics materials.